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In anthropology and demography, the human sex ratio is the ratio of males to females in a population. Like most sexual species, the sex ratio in humans is approximately 1:1. The sex ratio at birth is commonly thought to be 107 boys to 100 girls,〔(【引用サイトリンク】 url=https://www.cia.gov/library/publications/the-world-factbook/fields/2018.html )〕 although this value is subject to debate in the scientific community. The sex ratio for the entire world population is 101 males to 100 females.〔(【引用サイトリンク】 url=https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html )〕 Depending upon which definition is used, between 0.1% and 1.7% of live births are intersex.〔(【引用サイトリンク】title=How common is intersex? | Intersex Society of North America )〕 Gender imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants,〔Davis, Devra Lee; Gottlieb, Michelle and Stampnitzky, Julie; "Reduced Ratio of Male to Female Births in Several Industrial Countries" in ''Journal of the American Medical Association''; April 1, 1998, volume 279(13); pp. 1018-1023〕 war casualties, gender-selective abortions and infanticides, aging, and deliberate gendercide. More data are available for humans than for any other species, and the human sex ratio is more studied than that of any other species, but interpreting these statistics can be difficult. Human sex ratios, either at birth or in the population as a whole, are reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females. If there are 108,000 males and 100,000 females the ratio of males to females is 1.080 and the proportion of males is 51.9%. Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise. ==Natural ratio== In a study around 2002, the natural sex ratio at birth was estimated to be close to 1.06 males/female. In most populations, adult males tend to have higher death rates than adult females of the same age (even after allowing for causes specific to females such as death in childbirth), both due to natural causes such as heart attacks and strokes, which account for by far the majority of deaths and also to violent causes, such as homicide and warfare (for example, in the USA as of 2006, an adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2.5 to 3.5 times more likely to die in an accident than a female of the same age), resulting in higher life expectancy of females. Consequently, the sex ratio tends to reduce as age increases, and among the elderly there is usually an excess of females. For example, the male to female ratio falls from 1.05 for the group aged 15 to 65 to 0.70 for the group over 65 in Germany, from 1.00 to 0.72 in the USA, from 1.06 to 0.91 in mainland China and from 1.07 to 1.02 in India. In the United States, the sex ratios at birth over the period 1970–2002 were 1.05 for the white non-Hispanic population, 1.04 for Mexican Americans, 1.03 for African Americans and Indians, and 1.07 for mothers of Chinese or Filipino ethnicity. Among Western European countries ca. 2001, the ratios ranged from 1.04 in Belgium to 1.07 in Switzerland,〔(【引用サイトリンク】url=http://www.bfs.admin.ch/bfs/portal/en/index/themen/00/01/blank/02.html )〕 Italy,〔(【引用サイトリンク】url=http://www.un.org/esa/population/unpop.htm )〕 Ireland〔(【引用サイトリンク】url=http://data.un.org/Data.aspx?q=sex+ratio+birth&d=PopDiv&f=variableID%3a52 )〕 and Portugal. In the aggregated results of 56 Demographic and Health Surveys〔(Demographic and Health Survey )〕 in African countries, the ratio is 1.03, though there is also considerable country-to-country variation. Even in the absence of sex selection practices, a range of "normal" sex ratios at birth of between 103 to 108 boys per 100 girls has been observed in different economically developed countries,〔(【引用サイトリンク】url=http://data.un.org/Data.aspx?q=sex+ratio+birth&d=PopDiv&f=variableID%3a52%3btimeID%3a113&c=2,4,6,7&s=_crEngNameOrderBy:asc,_timeEngNameOrderBy:desc,_varEngNameOrderBy:asc&v=1 )〕 and among different ethnic and racial groups within a given country. In an extensive study, carried out around 2005, of sex ratio at birth in the United States from 1940 over 62 years,〔(【引用サイトリンク】url=http://www.cdc.gov/nchs/data/nvsr/nvsr53/nvsr53_20.pdf )〕 statistical evidence suggested the following: For mothers having their first baby, the total sex ratio at birth was 1.06 overall, with some years at 1.07. For mothers having babies after the first, this ratio consistently decreased with each additional baby from 1.06 towards 1.03. The age of the mother affected the ratio: the overall ratio was 1.05 for mothers aged 25 to 35 at the time of birth; while mothers who were below the age of 15 or above 40 had babies with a sex ratio ranging between 0.94 to 1.11, and a total sex ratio of 1.04. This United States study also noted that American mothers of Hawaiian, Filipino, Chinese, Cuban and Japanese ethnicity had the highest sex ratio, with years as high as 1.14 and average sex ratio of 1.07 over the 62-year study period. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Human sex ratio」の詳細全文を読む スポンサード リンク
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